Chrome Versus Chromium

Chromium is the bleeding edge development of the project. It is the browser, and whatever its current state may be. We usually get a few Chromium builds per day.

Google Chrome is simply a "rebranding" of Chromium, but is a little more ready for public consumption.

Stable: The, well, stable version of Chrome, which is absolutely ready for public consumption. Updates about every 6 months.

Beta: Still relatively stable, but you may see new, untested features in it. No guarantees. Updates every few weeks (I think).

Dev: Think of it as the Beta version, except it's the boiling pot of new features/testing. Absolutely no guarantees.  Updates weekly.

When the developers feel Chromium is in a ready enough state, they'll release updates to the Stable, Beta, and Dev branches, respectively.

http://code.google.com/p/chromium/wiki/ChromiumBrowserVsGoogleChrome